At present, recording media based upon the magneto-optical recording technique have widely been put into practical use. In the magneto-optical recording method, the magneto-optical effect of a thin film formed of a heavy rare earth element--transition metal alloy of perpendicular magnetic anisotropy is utilized for realization of practical rewritable recording media of a large capacity. The recording medium of this type is of portability, so that there exists such merit that recorded information can be kept secret or recorded information of large capacity can be moved easily.
With the advance of the development of multimedia for recording image data for instance, further increased memory capacitance is required more and more for the magneto-optical recording media. For satisfying this requirement, at present the higher density technique is now being researched on the basis of a laser beam of shorter wavelength (400 to 700 nm). One of the technical problems related to this technique is to obtain magnetic material which has a large Kerr rotational angle in as short a wavelength range as 400 to 700 nm. One of these materials is amorphous alloy which contains a light rare earth element (e.g., NdFeCo) and a transition metal. In the amorphous alloy, since the 4 f electrons contribute to an increase in the magneto-optical effect, the Kerr rotational angle is large in the short wavelength range (J. Mag. Soc. Jpn. 11 Suppl. Sl. 273 (1991)), so that this alloy is an effective candidate for the high density recording media.
In the NdFeCo, since the magnetization is large, there exists a problem in that it is difficult to form the perpendicular magnetized film required for the magneto-optical medium. To overcome this problem, recently there has been proposed such a method that a NdCo layer which has large Kerr rotational angle in a short wavelength range (not formed into a perpendicular magnetized film) is sandwiched between two TbFeCo layers (formed into perpendicular magnetized films) for providing mutual exchange coupling so that the three magnetic layers can function as a perpendicular magnetized film (Journal of Applied Physics Vol. 69 p4761, by Iiyori et al. of IBM Japan). Hereinafter, the structure such that a non-perpendicular magnetized layer is sandwiched between the two perpendicular magnetized layers as described above is referred to as sandwich structure.
In the prior art sandwich structure such that the NdCo layer is sandwiched between the two TbFeCo layers, however, since laser beam signals must be detected from the NdCo layer through the TbFeCo layer with an about 100 angstrom thickness, there exists a problem in that a sufficiently large Kerr rotational angle is not obtained in the case of a laser beam in the short wavelength range (400 to 700 nm).
On the other hand, in order to provide a large Kerr rotational angle in response to a laser beam, a reflection structure is so far well known. In this structure, a light reflection layer formed of material such as Al is formed in the recording medium, and a magnetic layer for producing a large Kerr rotational angle is formed on the surface of the light reflection layer. A laser beam outputted from an optical head is passed through the magnetic layer, reflected from the reflection layer, and again passed through the magnetic layer, before received by the optical head. In this method, since the laser beam is passed through the magnetic layers twice, it is possible to provide a large Kerr rotational angle to the laser beam. In the prior art sandwich structure as already described, however, since the magnetization of the middle non-perpendicular magnetized film must be stood in the vertical direction by the strong exchange coupling force applied from the perpendicular magnetized films provided on both the sides, the thickness of at least one side perpendicular magnetized film must be as thick as about 1000 angstrom. As a result, in the prior art sandwich structure, it is impossible to transmit a laser beam therethrough, and thereby it is impossible to combine the sandwich structure with the afore-mentioned reflection structure.